A detonator is usually used to cause detonation of a large explosive charge, even though detonators themselves, in some cases, can be used as charges. In blasting operations, bores are drilled in which explosives are applied to create a charge. A detonator is arranged in or adjacent to the explosive, which is set off by the intermediary of the detonator.
In large blasting operations, it is often desirable to create a carefully calculated delay of the detonation of the detonators for the purpose of obtaining a desired blasting sequence. The delay is achieved in various ways depending on the type of detonator being used. A detonator may, for instance, be pyrotechnic or electronic. In pyrotechnic detonators, the desired detonation delay is achieved by means of delay charges, which have a preset burning time. In electronic detonators, the desired delay is usually achieved by means of electronic circuits, which count down a programmed delay time and then feed a current to an electric fuse head, which causes the detonator to detonate.
In many cases, it is desirable to be able to walk around the blast site for inspection and monitoring purposes before firing. Furthermore, it may be desirable to be able to initiate firing from any optional site remote from the blast site.
One problem associated with detonator systems according to prior art is that the operator has to initiate firing from a site adjacent to the blast site to be able to physically press the arming and firing buttons of a blasting machine. To set off the detonators, the blasting machine is connected thereto by means of, for example, a fuse or electrical wires. Wireless firing would allow a more flexible system.
Wireless firing of detonators is disclosed, for instance, in U.S. Pat. No. 5,159,149. The purpose of the detonator system described therein is to get round the use of any physical connection between the detonators in a round. According to the description, this purpose is achieved by providing each detonator of the round with a receiver for receiving a fire command signal from a transmitter. When preparing a blast, a transportable charging and programming unit is carried round the blast site, said unit being connected to the respective detonators for charging electric fuse means and for programming the desired delay times in the detonators.
However, the detonator system of the above patent specification has a number of serious limitations and drawbacks. Although the physical interconnection of detonators by means of a fuse or electrical wires has been eliminated, a charging unit that has to be carried round the blast site is required. It is evident that this will complicate the work to be carried out. A further drawback of such a method is that the state of the detonators cannot be checked when the programming unit is disconnected. Consequently, it is not possible to ensure, at a later stage, that the detonators are charged and that they will in fact detonate when receiving a fire command.
Moreover, a system according to the above patent specification is associated with considerable risks. In reality, the person preparing the charges at the blast site will be surrounded by armed detonators, which will detonate upon receipt of a fire command. Any radio frequency interference in the receiver of a detonator, or any inadvertent transmission of a fire command from the transmitter, would therefore have devastating consequences. It is even conceivable that another radio signal source not associated with the detonator system transmits, intentionally or by mistake, a radio signal that is interpreted as a firing signal by the receiver.
Furthermore, each individual detonator has to be provided with a receiver. Considering the fact that a round may comprise a considerable number of detonators, this may imply a significant increase in system costs.
Another general problem associated with wireless firing of detonators is that it is necessary to somehow ensure that only the intended round can be fired. For example, a plurality of rounds may be arranged within a limited area (within one radio coverage area), but it may be desirable to fire only one of these rounds. The obvious problem then is how to make sure that only the intended round is fired.
Accordingly, there is a demand for improved detonator systems that eliminate the risks and problems mentioned above.